The present invention is applicable for inverter-type power plants where the electric distribution line is used as stand-by unit. The present invention is composed of a ballast impedance and an electric mesh comprising an energizing circuit of a first section suitable for insulating a power plant from the distribution line; the contacts of a second section which are monitored by a supervisory control circuit of the power plant; and an electric probe making available a voltage or a current measurement when current flows through the distribution line.
The present invention is directed to a circuit for inhibiting voltage and current returns to the distribution line in an inverter-type power plant which uses the distribution line as stand-by unit. In this system, the load is switched during the presence of faults in one or more inverters or during the presence of an overload condition caused, for example, by a fault in the utilizing units.
As shown, the inverters receive a direct voltage at their input and output an alternating voltage which feeds a load. The power plants of this type comprise one or more inverters, connected in parallel, as well as, one or more stand-by inverters or electric distribution lines as stand-by units. The power plants are then used to feed systems which require an high degree of reliability. It is therefore necessary to provide working and stand-by devices, as well as, a supervisory control circuit. The present invention is to be utilized when the electric distribution line is used as stand-by unit.
In particular, when the supervisory control circuit detects the presence of faults in the working devices, this control circuit causes a disengagement of a first set of static switches which, by assuming a normal condition of "open", cause a sectioning of the connection between the load and the working devices. The static switches may be centralized (located on the multiple unit which connects in parallel the inverters to the utilizing units) or remotely located (i.e. in cascade to each inverter). At the same time the supervisory control circuit releases a second set of static switches which, by assuming a normal condition of "closed", connect the load to the distribution line which is the stand-by unit, as specified above. During normal operations (the load being fed by inverters) and due to a fault in the system, it is possible that the second set of static switches unduly assume the configuration of "closed", so that the distribution line is connected to the inverters. The fault causes both accident prevention and handling problems. The problems relating to the accident prevention arise when the faults mentioned above occur, and a determined voltage or current returns to the electric distribution line. When the faults occur during a maintenance operation (subsequent, for example, to a sectioning operation of a network section which is therefore insulated from the power generators located upstream from the network section interested), problems related to the accident prevention may arise, as the personnel works with the understanding that the mains are not connected to any power source. These handling problems arise in conditions similar to the above (load feeded by inverters and with a fault in the second set of static switches), and they are caused by a power transfer to the distribution line in the absence of any regulations suitable to control the transfer.
The units which arrange the electric energy distribution do not usually allow the direct parallel connection between the network and a set of inverters, except when specific regulations are provided and adequate devices for the execution of the operation are arranged.
In the known power plants, due to faults in the static switches or in the supervisory control circuit, the voltage and current returning to the electric distribution line is inhibited by a current return device connected in series to the electric distribution line. As known, these devices are adequate to carry out a sectioning of the line when they detect an energy transfer in the direction opposite to the normal working direction. A current return device, namely "a reverse power flow blocking and sensing device" is shown in the Figures of U.S. Pat. No. 4,104,539 and disclosed in column 6, lines 60 to 66 of this U.S. Patent. These devices are too expensive and have too large of an operating time. They are, therefore, suitable only to solve the handling problems cited above, but are not adequate to solve the problems associated with accident prevention, as personnel usually intervene when the sectioning has been already carried out. No current, therefore, flows, and the current return device may not operate. In fact, since the operating time of these devices is too long, the sectioning time of the connection may be not effected (if the current intensity of the human body is inferior to the operating current) or may be effected when the accident has already occurred.
The known devices are not capable of solving the problems of an accident prevention nature caused by voltage returns to the electric distribution line.